More than one third of the population in rural and in peri-urban areas of developing countries have no access to sufficient or clean drinking water free of pathogens. Thus, waterborne gastroenteritis remains a major infrastructural and public health problem particularly, as effective treatment (filtration, chlorination, treatments plants) is often beyond financial means or environmental resources used for water purification (fire-boiling, burning carbon-based fuels) become scarce in those communities. In this context solar disinfection of drinking water is especially appealing using a combination of irradiation by direct sunlight and solar heating to kill the water-borne pathogens in contaminated drinking water. To date, the efficacy of the SODIS technology as a home-based, low-cost intervention to provide safe drinking water in low income countries is well established, and a large-scale promotion and dissemination program is under way in seven Latin American countries. The principal objective of this study is to evaluate the effectiveness of home-based solar water disinfection (SODIS) in reducing the burden of gastrointestinal illness in children under 5 years in rural villages participating in a country-wide Bolivian SODIS program. We will conduct a community (cluster)-randomized controlled trial following a cohort of children under age 5 in each community. Totally, 22 communities will be selected from among those districts designated by the country-program to receive the SODIS intervention. A pair-matched design will be employed where communities are first ranked according to their baseline incidence of diarrheal disease and the intervention then assigned within each of the 11 consecutive pairs of communities randomly to one of them. In each cluster, 30 children (660 in total) will be enrolled and followed up for 12 months. Data on diarrheal illness will be obtained from morbidity diaries kept by mothers and validated through weekly home visits. Stool samples will be collected during the baseline morbidity surveys and at times of a diarrheal episode in a child during follow-up. Water quality monitoring of raw water sources used for drinking water and of water samples after treatment with the SODIS device will be conducted systematically. Mothers of participating children will be interviewed at baseline and during the trial with regard to current water use, behavioral and environmental exposures of their child in the home and within the community. This study will specifically estimate: (1) how much of the efficacy of the SODIS technology established in laboratory experiments and in two tightly controlled phase-Ill trials can be retained as effectiveness, i.e., under program conditions; (2) the preventive fraction of all-cause child-diarrhea attributable to SODIS; and (3) pathogen-specific attributable risks of diarrheal illness and the cost-effectiveness of SODIS will be calculated. The project is organized by the University of California, Berkeley, with its substantial experience in water intervention trials in U.S. and it benefits from the tradition of North-South collaboration in public health research of the Swiss Tropical Institute, Basel, Switzerland. It is run jointly with the Bolivian partners at the Universidad Mayor de San Simon and the Fondazion SODIS that coordinates the Bolivian SODIS program.